Modulation of electromagnetic radiation has been accomplished in the past either by mechanical means, or by electro-optical and magneto-optical means. Mirrors mounted on appropriate electroacoustical transducers have also been used for light modulation. The prior art has a number of intrinsic shortcomings which the instant invention resolves. Mechanical modulation has usually involved the rotation of mirrors or their translational movements, or the use of mechanical shutters and choppers. Due to the mechanical nature of these devices, major limitations on speed of operation as well as limitations on available size have existed. For these reasons, electroacoustical transducers on which mirrors are mounted (or deposited) are rapidly replacing mechanical modulation in many of the more demanding applications.
Electro-optical and magneto-optical devices are electronically controllable and thus perform intrinsically faster than mechanical means of optical modulation. The devices act mostly on light transmitted through such devices by modulating the index of refraction of the electro-optical or magneto-optical crystals therein incorporated. As a result, both families of devices absorb an appreciable amount of the impinging light in any of their states, and their operation, particularly in high energy density applications (lasers) is hampered by optical and heating losses.
The loss problem is less severe in traditional mirrors, and thus for high energy density applications (like lasers) mechanically modulated mirrors are preferred. Nevertheless, even the best traditional mirrors, having the best normal conductors (gold, silver, copper and aluminum) as reflecting surfaces, absorb close to one percent of the incident light. This shortcoming, coupled with restrictions imposed by the mechanical nature of modulation, limits the application of traditional mirror technology when the reflected beams, such as those found in laser technology, have extremely high energy densities.
Classical superconductors have never been considered for mirror and other reflective applications despite their having conductivities which are better than those of normal conductors (such as gold, silver and copper). The reason is rooted in the simple fact that in known superconductors the high conductivity is due to pairing of charge carriers. This pairing involves a binding energy that for most classical superconductors is less than 3 milli-electronvolts. When electromagnetic radiation with any wavelength shorter than about 0.4 mm impinges on such classical superconductors, it is absorbed and in the process decouples the paired charge carriers. For most optical applications, including the infrared wavelengths, superconductors with much higher pairing energies are needed in order to be able to reflect at much shorter wavelengths.
Until recently, it was believed that superconductivity above 23.degree. K., and therefore band gaps in excess of 3 milli-electronvolts were not possible. This belief was rooted in the theoretical work now named the BCS theory (Bardeen, Cooper and Schrieffer) which predicted such an upper limit. As a result, no research in the field of superconducting mirrors and reflectors can be cited by me.
The temperature at which superconductivity may occur in a superconductor (in the absence of any external magnetic fields) is termed the critical temperature of that superconductor and this term will be used herein.
In the early 1970's a number of theoretical proposals were presented, suggesting that the critical temperature for superconductivity could be increased. (V. L. Ginzburg, Usp. Fiz. Nauk. 101, 185 (1970)) (D. Allender, J. Bray, J. Bardeen, Phys. Rev. B8, 4433 (1973)). A significant experimental breakthrough in high temperature superconductivity (critical temperatures in excess of 23.degree. K.) was provided in November 1986 by Bednorz and Muller when they published a tentative disclosure of high temperature superconductivity (Georg Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)). Rapid confirmation by others soon occurred. For instance, a report cites a critical temperature above 30.degree. K. for La.sub.(2-x) Ba.sub.(x) CuO.sub.(4-y), (H. Takagi, S. Uchida, K. Kitazawa, S. Tanaka, Jpn. J. Appl. Phys. 26, L123 (1987)). Confirmation of a critical temperature of 93.degree. K. was reported by Chu for an yttrium-barium-copper oxide ceramic (M. K. WU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, Mar. 2, 1987, p. 908.) This material was dubbed the 123 compound and has served as a model for advanced research in the field.
During 1987 and 1988, a number of families of high temperature superconductors were discovered with confirmed critical temperatures all the way to 162.degree. K. These materials were usually ceramics containing copper (whose apparent valence state appears to be trivalent), an alkaline metal (Ca, Sr, or Ba) and a rare earth, including yttrium. Most of these superconductors showed some degree of anisotropy in their properties and it was therefore significant when a cubic ceramic with a critical temperature above 23.degree. K. (specifically 30.degree. K.) was discovered based on a complex oxide of Ba, K and Bi. This superconductor was the first high temperature superconductor without copper in its composition, thus indicating that the occurrence of high temperature superconductivity may be more prevelent than had been realized heretofore. Additionally, amorphous high temperature superconductors have also been reported (based on the bismuth compounds in which some of the bismuth was replaced with lead). The critical temperatures and critical current of these amorphous superconductors are somewhat lower than those of their crystalline counterparts.
There are some scattered reports of superconductivity above 162.degree. K. For instance, R. G. Kulkarui has reported superconducting oxides having an approximate composition CaO.sub.(0.5) ZnO.sub.(0.5) Fe.sub.2 O.sub.4. Ogushi reported superconductivity at room temperature in yet ill-defined niobium strontium lanthanum oxides. While these reports have yet to be confirmed independently by other researchers, it is reasonable to expect superconductors having critical temperatures near to room temperature, with much higher electron-pair binding energy to become available in the near future.
In classical BCS superconductors, the optical band gap is equal to the electronic band gap (as measured on a Josephson junction) and is about 3.5 kT.sub.c. However, in some of the high temperature superconductors, I have determined that observed critical temperature and the "optical" critical temperature are not equal, and that the optical critical temperature, which I have termed the "virtual critical temperature" can be much larger than thermodynamic critical temperature. As a result, the optical band gap can be much higher than 3.5 kT.sub.c. I further find that for wave lengths longer than their optical band gap, selected superconductors reflect electromagnetic radiation more efficiently than normal metals, and that the absorption losses may be as much as three to four orders of magnitude lower than in gold.
I also find that when the superconductors are quenched by selected means into their normal state, they combine absorption, reflection and transmission of light at ratios that depends on the physical properties of their respective normal states. Thus, superconducting materials will be found to have charge carriers with virtual binding energy in excess of 2 electron-volts. This makes possible mirrors capable of reflecting electromagnetic radiation in the infrared as well as in the visible part of the spectrum. This postulation is based not only on the classical scaling of charge carriers binding energy with the thermodynamic critical temperature (3.5 kT.sub.c) (which would be insufficient to reach the infrared part of the spectrum unless critical temperatures of superconductors in excess of 500.degree. K. are achieved), but particularly on my discovery that virtual critical temperatures can be more than twice the thermodynamic critical temperatures.
A further tenet is that some of these high temperature superconductors may have the unique property that, in their normal state, they are insulators and basically transmit electromagnetic radiation, at least within a specific band that depends on the electronic state of the normal state in a manner well known in the art. To differentiate between the different classes of superconductors and clearly define classes that are suitable for the practice of this invention, I have classified superconductors according to the nature of their corresponding normal state. Classical BCS superconductors are usually metallic in their normal state and therefore belong to a class I have denoted as (SC,M), namely, that below and above their critical temperatures, a superconducting and a normal metal phase exists, respectively. A few examples of this class of superconductors are mercury, niobium and its A15 intermetallic compounds with tin and germanium.
The new class of superconducting oxides, that are semimetals or semiconductors in their normal state, thus belong to a class that I denote as (SC,S) in a similar manner. Examples of superconductors belonging to this class are the 123 compounds and the bismuth based oxide superconductors. Finally the last class of superconductors of my new invention are insulators in their normal state and thus belong to a class that I denote as (SC,I). An example of this group is Kulkarui's superconducting spinel like compound with an approximate composition CaO.sub.(0.5) ZnO.sub.(0.5) Fe.sub.2 O.sub.4.
I have determined that some superconductors belonging to the last two classes, have virtual critical temperatures that are higher than the actual, or thermodynamic critical temperature. I have further found that the differential is much larger for the superconducting oxides of the class (SC,I) with a normal insulating state. Thus, the present invention concerns only switchable mirrors made with, or of superconductors belonging to the (SC,S) class or the (SC,I) class, wherein the normal states are either semi-conducting (or semi-metal) or insulating, respectively.
I have found that such mirrors behave like perfect mirrors, at least to the wavelength energies which are equal to the binding energy of the superconducting charge-carrying pairs. While in the quenched phase, said mirrors combine absorption, reflection and transmission of light at ratios which depend on the physical properties of the normal state.
The classical superconductors (belonging to the class (SC,M), where the normal state is a normal metal) cannot be used as controllable mirrors because the electron pairing energy of these materials is too low. The main interaction between these superconductors and electromagnetic radiation is absorption, resulting in de-pairing of the superconducting electrons, provided the wavelengths are shorter than about 0.4 millimeter.